Thin-film transistors (TFTs) with amorphous indium gallium zinc oxide (a-IGZO) active layers were fabricated using radio-frequency sputter deposition with variable dc substrate bias. High substrate bias sputtered a-IGZO TFTs produce more carriers in an a-IGZO layer and result in a negative gate threshold voltage (V-T) shift, an increase in the field-effect mobility (mu(FE)), and an increase in the off-state current (I-off). The subthreshold gate swing (S) is degraded with an increased substrate bias. The different TFT characteristics as a function of substrate bias can be attributed to energetic ion bombardment during a-IGZO growth.